A carboxyl group-containing water-soluble polymer is generally manufactured by carrying out precipitation polymerization in an inert solvent in which a monomer component containing an α,β-unsaturated carboxylic acid and optionally a polymerizable compound having two or more ethylenically unsaturated groups is dissolved but in which a carboxyl group-containing water-soluble polymer produced by polymerization of the monomer component is not dissolved. Precipitation polymerization refers to a polymerization method wherein a polymer produced by a polymerization reaction is precipitated out of a solvent by using an inert solvent as described above, by which polymerization method a carboxyl group-containing water-soluble polymer can be obtained in a form of a powder.
In a general manufacturing of a carboxyl group-containing water-soluble polymer by precipitation polymerization, a polymerization reaction is progressed by dissolving a required monomer component in an inert solvent charged in a reaction vessel, and heating the reaction system in the presence of a radical polymerization initiator (for example, see Patent Literatures 1, 2, 3, 4 and 5). However, since this method tends to result in very high viscosity of a reaction solution slurry formed with the progress of the polymerization reaction, it is difficult during the polymerization reaction to control heat of reaction, and there is a risk of causing bumping of a reaction solution and a possibility of having an adverse effect on the degree of polymerization or physical properties of a produced polymer. In particular, when the concentration of a monomer component in the reaction system exceeds 10% by volume, this tendency is remarkable.
Thus, in manufacturing of a carboxyl group-containing water-soluble polymer by general precipitation polymerization as described above, although increase in viscosity of a reaction solution slurry is suppressed by reducing the concentration of a monomer component in an inert solvent to cope with the above-mentioned defect, it is difficult to increase production efficiency of a carboxyl group-containing water-soluble polymer due to decrease in the amount of production per batch.
As other methods, for manufacturing a carboxyl group-containing water-soluble polymer, Patent Literatures 6 and describe methods of carrying out a reaction by additionally adding a monomer component to a reaction solution in which a specific amount of a monomer component remains unreacted. In addition, Patent Literature 8 (in particular, claims and paragraph 0032) describes a method of dividedly adding a monomer component to an inert solvent in 2 to 8 additions and a method of continuously adding a monomer component little by little to an inert solvent.
However, methods of additionally adding or dividedly adding a monomer component lack practicality since an effect of suppressing increase in viscosity of a reaction solution slurry is insufficient despite complication in control and operation. In addition, there is a tendency that the produced carboxyl group-containing water-soluble polymer becomes light and small in the bulk specific gravity. Furthermore, a method of continuously adding a monomer component lacks practicality since the optimal range of reaction conditions such as polymerization temperature is limited and thus the control is difficult as described in Patent Literature 8.